When following someone on a different server on the Fediverse, the remote server decides whether you are allowed to do so. This enables features like private accounts. Due to an implementation mistake, Pixelfed ignores this and allows anyone to follow even private accounts on other servers. When a legitimate user from a Pixelfed instance follows you on your locked fediverse account, anyone on that Pixelfed instance can read your private posts. You don’t need to be a Pixelfed user to be affected.

Pixelfed admins should update to v1.12.5 ASAP, but upgrading can be a major hurdle.

Importantly, your Mastodon or GoToSocial instance isn’t handing your private posts to any random server, just because it asks. The problem only becomes apparent when you have at least one legit accepted follower from a Pixelfed server. Now that server is allowed to fetch all your private posts. And when it knows the posts, it has to decide who to show them. When you accept a follower, you not only place your trust to keep a secret on them, but also on their admin and the software they are running.

Edited to add the last block quote.

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[–] fmstrat@lemmy.nowsci.com 1 points 5 days ago (1 children)

The whole point of this issue with Pixelfed is that none of what you describe is required.

Find any follower of a Fediverse account of any kind (Target Account) that's on a Pixelfed server. Go to that Pixelfed server, view "private" posts from Target Account there.

No need to set up a server, or get sent anything. Granted, even without this flaw ActivityPub is not the way to go for anything private.

[–] PhilipTheBucket@ponder.cat 1 points 5 days ago

even without this flaw ActivityPub is not the way to go for anything private.

This is the real issue. The whole story about how his partner's posts were getting shown to random people should have ended with both of them realizing that these posts were in no reliable way "private," and to stop putting them up with the assumption that they would be. Not with them yelling at Pixelfed for the way it works, and then yelling at Pixelfed again for starting to honor these fake privacy settings.